1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a stepping motor used for a carriage driving mechanism of an optical pickup such as a pickup for a CD player and the like.
2. Description of the Related Art
A CD player or the like has a device for moving a carriage of an optical pickup on a recording medium shaped like a disc, such as a CD (Compact Disc) and the like. In such a CD player, in order to move the carriage in the radial direction accurately and quickly, a positioning control is performed, and a stepping motor is used for the positioning control.
Furthermore, a trapezoidal shape driving method is typically used for the positioning control. As shown in FIG. 6, according to the trapezoidal shape driving method, acceleration and deceleration are rectilinearly performed, and a constant velocity period is set between an accelerating period and a decelerating period.
In FIG. 6, an abscissa is the number of steps of the stepping motor from a present position, and an ordinate is the response frequency of the stepping motor. In FIG. 6, "fs" means a maximum response frequency, and "f.sub.0 " means a self-starting frequency.
The acceleration and the deceleration are realized by changing the response frequency of the stepping motor, i.e., an interval of a pulse to be used for exciting the stepping motor, under control of a CPU installed in a control device. The constant velocity driving is realized by making the interval of the pulse constant.
As shown in FIG. 6, the acceleration is performed by increasing the response frequency as high as possible, and then, the constant velocity driving is performed, thereafter, the deceleration is performed by decreasing the response frequency in the same rate as the acceleration. According to this manner, it is possible to move the carriage to a target position quickly.
As an excitation method for driving the stepping motor, a two-phase excitation method can be used. If the two-phase excitation method is used for driving the stepping motor, it is possible to make the response frequency high. Therefore, the two-phase excitation is preferably used for a high velocity driving. However, in the two-phase excitation, as the range of movement corresponding to one pulse is large, a large disturbance arises in an actuator for holding an objective of the optical pickup with aid of a spring. Such a large disturbance is adversely affect the positioning control. Accordingly, the one-two-phase excitation method, which is an excitation method in the middle of the one-phase excitation and the two-phase excitation, is preferably used for the positioning control.
Here, the positioning control using the one-two-phase excitation according to the trapezoidal shape driving method is explained with reference to FIG. 6.
In order to move the carriage from a present position to a target position, at first, a distance between the present address to the target address is calculated, and the total number of steps is calculated on the basis of the calculated distance. In case that 1 step in a microstep method is used as the unit of counting the number of steps, the total number of steps Sa is obtained by dividing the calculated distance by a distance of movement corresponding to 1 step in the microstep method.
Next, the position of the rotor of the stepping motor is adjusted by using the microstep method in order to move the rotor position to a one-two-phase excitation position before the acceleration is performed by using the one-two-phase excitation method. Actually, the number of steps Sb corresponding to a distance from the present position to the one-two-phase excitation position closest to the present position is calculated.
Furthermore, a position adjustment called the landing is performed between the end of the deceleration and the stopping. The number of steps Sc necessary for the landing is set in advance.
Then, in case that 1 step in the one-two-phase excitation method corresponds to 8 steps in the microstep method, the number of the steps Sd necessary for the acceleration, the deceleration and the constant velocity driving is calculated by the following expression. EQU d=(a-(b+c))/8
Next, the number of steps Sd is divided by three to thereby calculate the number of steps Se necessary for the acceleration, the number of steps Sf necessary for the constant velocity driving and the number of steps Sg necessary for the deceleration respectively. When it is determined that the response frequency exceeds the maximum frequency fs if the acceleration is performed by the number of steps Se, the number of steps S1 is set as the number of steps Se and the number of steps Sg respectively. Here, the "S1" is the number of steps required to increase the response frequency to the maximum response frequency fs. In this case, the number of steps "d-(e+g)" is set as the number of steps Sf.
Thereafter, as shown in FIG. 6, the stepping motor is driven on the basis of the numbers of steps Sb, Se, Sf, Sg and Sc. Namely, at first, the rotor of the stepping motor is moved by the microstep method until the number of steps reaches the number of steps Sb. Next, the stepping motor is driven by the one-two-phase excitation method until the number of steps reaches the number of steps Se. During this period, the stepping motor is accelerated according to the predetermined acceleration. Next, the stepping motor is driven by the one-two-phase excitation until the number of steps reaches the number of steps Sf. During this period, the stepping motor is rotated in a constant velocity. Next, the stepping motor is driven by the one-two-phase excitation method until the number of steps reaches the number of steps Sg. During this period, the stepping motor is decelerated according to the predetermined deceleration. Next, the stepping motor is driven by the microstep method until the number of steps reaches the number of steps Sc, in order to perform the landing. Then, the stepping motor is accurately stopped at the target position.
In the above mentioned operation, in the acceleration, the deceleration and the constant velocity driving, the stepping motor is always driven by the one-two-phase excitation method. Therefore, there is a problem that it is impossible to make torque high and it is impossible to make maximum velocity high.
Namely, in the case that the stepping motor is driven by the one-two-phase excitation, there is a difference between torque generated by the one-phase excitation and torque generated by the two-phase excitation, and thereby, the torque frequently changes. Therefore, in case that the one-two-phase excitation is used for driving the stepping motor, it is necessary to make the maximum response frequency lower due to dispersion of a load, as compared with the maximum response frequency in the case that the two-phase excitation is used.
On the other hand, if the stepping motor is always driven by using the two-phase excitation in the acceleration, the deceleration and the constant velocity driving, since the range of movement corresponding to one pulse is large, a large disturbance arises in an actuator for holding an objective of the optical pickup, as mentioned above.